Process and plant for low-temperature fractionation of air

ABSTRACT

A SPECTRA process for low-temperature fractionation of air, in which bottoms liquid from an additional second rectification column used to obtain oxygen is evaporated in a second condenser-evaporator. In this second condenser-evaporator, gas that has been evaporated beforehand in a first condenser-evaporator, which is used for condensation of tops gas from a first rectification column, is condensed at the pressure level of the previous evaporation. The invention likewise provides a corresponding plant.

The invention relates to a process for the low-temperature fractionation of air and to a corresponding plant in accordance with the preambles of the independent claims.

PRIOR ART

The production of air products in the liquid or gaseous state by low-temperature fractionation of air in air fractionation plants is known and described, for example, in H.-W. Häring (editor), Industrial Gases Processing, Wiley-VCH, 2006, in particular Section 2.2.5, “Cryogenic Rectification.”

Air fractionation plants have rectification column systems which, for example, can conventionally be designed as two-column systems, in particular as classical Linde double-column systems, but also as triple-column or multi-column systems. In addition to the rectification columns for extracting nitrogen and/or oxygen in the liquid and/or gaseous state, i.e., rectification columns for nitrogen-oxygen separation, rectification columns for extracting further air components, in particular the noble gases krypton, xenon, and/or argon, can be provided. Frequently, the terms “rectification” and “distillation” as well as “column [Säule]” and “column [Kolonne]” or terms composed therefrom are used synonymously.

The rectification columns of the mentioned rectification column systems are operated at different pressure levels. Known double-column systems have what is known as a high-pressure column (also referred to as a pressure column, medium-pressure column, or lower column) and what is known as a low-pressure column (also referred to as an upper column). The high-pressure column is typically operated at a pressure level of 4 to 7 bar, in particular approximately 5.3 bar. The low-pressure column is operated at a pressure level of typically 1 to 2 bar, in particular approximately 1.4 bar. In certain cases, even higher pressure levels may be used in either rectification column. The pressures cited here and below are absolute pressures at the top of the respective columns indicated.

So-called SPECTRA processes are known from the prior art for providing pressurized nitrogen as the main product. They are explained in more detail below. In a SPECTRA process, a so-called oxygen column, which can be operated at or above the pressure level of a typical low-pressure column, can be used to obtain pure or high-purity oxygen. This low-pressure column is present in addition to the rectification column used for nitrogen extraction and is fed therefrom.

According to EP 1 995 537 A2, feed air is cooled in a main heat exchanger and introduced into a single column for nitrogen extraction. A nitrogen product stream is taken from the upper region of the single column. A first residual fraction is taken from the lower or central region of the single column, recompressed and subsequently fed back into the single column. An oxygen-containing stream is taken from the single column at an intermediate point and fed to a pure oxygen column. A pure oxygen product stream is taken in the liquid state from the lower region of the pure oxygen column. The pure oxygen product stream is evaporated and heated in the main heat exchanger against feed air and finally obtained as a gaseous product.

U.S. Pat. No. 6,279,345 B1 relates to an air fractionation system that can be used for producing ultra-high-purity nitrogen or ultra-high-purity oxygen, wherein oxygen-enriched liquid in a rectification column is evaporated in two steps using a split head condenser, and vapor from the first step is compressed and returned to the rectification column.

The object of the present invention is to improve a SPECTRA process with corresponding oxygen extraction, primarily with regard to energy consumption and material yield.

DISCLOSURE OF THE INVENTION

Against this background, the present invention proposes a process for the low-temperature fractionation of air and a corresponding plant with the features of the independent claims. Preferred embodiments form the subject-matter of the dependent claims and of the following description.

Prior to explaining the features and advantages of the present invention, some of the principles of the present invention are explained in greater detail and terms used below are defined.

The devices used in an air fractionation plant are described in the cited technical literature, for example in Haring (see above) in Section 2.2.5.6, “Apparatus.” Unless the following definitions differ, reference is therefore explicitly made to the cited technical literature with respect to terminology used within the framework of the present application.

Liquids and gases may, in the terminology used herein, be rich or poor in one or more components, wherein “rich” can refer to a content of at least 75%, 90%, 95%, 99%, 99.5%, 99.9%, or 99.99%, and “poor” can refer to a content of at most 25%, 10%, 5%, 1%, 0.1%, or 0.01% on a mole, weight, or volume basis. The term “predominantly” can correspond to the definition of “rich.” Liquids and gases may also be enriched in or depleted of one or more components, wherein these terms refer to a content in a starting liquid or a starting gas from which the liquid or gas has been extracted.

The liquid or the gas is enriched if it contains at least 1.1 times, 1.5 times, 2 times, 5 times, 10 times, 100 times, or 1000 times the content, and depleted if it contains at most 0.9 times, 0.5 times, 0.1 times, 0.01 times, or 0.001 times the content of a corresponding component, based on the starting liquid or the starting gas. If, for example, reference is made here to “oxygen,” “nitrogen,” or “argon,” this is also understood to mean a liquid or a gas which is rich in oxygen or nitrogen but need not necessarily consist exclusively thereof.

The present application uses the terms “pressure level” and “temperature level” to characterize pressures and temperatures, which means that corresponding pressures and temperatures in a corresponding plant do not have to be used in the form of exact pressure or temperature values in order to realize the inventive concept. However, such pressures and temperatures typically fall within certain ranges that are, for example, ±1%, 5%, or 10% around an average. In this case, corresponding pressure levels and temperature levels can be in disjointed ranges or in ranges that overlap one another. In particular, pressure levels, for example, include unavoidable or expected pressure losses. The same applies to temperature levels. The pressure levels indicated here in bar are absolute pressures.

Where “expansion machines” are mentioned here, these refer to typically known turboexpanders. These expansion machines can, in particular, also be coupled to compressors. These compressors may in particular be turbocompressors. A corresponding combination of turboexpander and turbocompressor is typically also referred to as a “turbine booster.” In a turbine booster, the turboexpander and the turbocompressor are mechanically coupled, wherein the coupling may take place at the same rotational speed (for example via a common shaft) or at different rotational speeds (for example via suitable gearing). In general, the term “compressor” is used herein. Here, a “cold compressor” refers to a compressor to which a fluid stream is supplied at a temperature level significantly below 0° C., in particular below −50, −75, or −100° C. and up to −150 or −200° C. A corresponding fluid stream is cooled to a corresponding temperature level in particular by means of a main heat exchanger (see below).

A “main air compressor” is characterized in that it compresses all of the air supplied to the air fractionation plant and separated there. In contrast, in one or more optionally provided further compressors, for example booster compressors, only a portion of this air that has already been previously compressed in the main air compressor is further compressed. Accordingly, the “main heat exchanger” of an air fractionation plant represents the heat exchanger in which at least the predominant part of the air supplied to the air fractionation plant and separated there is cooled. This takes place at least in part and possibly only in counterflow to material streams that are discharged from the air fractionation plant. In the terminology used herein, material streams or “products” “discharged” from an air fractionation plant are fluids that no longer participate in circuits within the plant but are permanently removed therefrom.

A “heat exchanger” for use in the context of the present invention can be designed in a manner customary in the art. It serves for the indirect transfer of heat between at least two fluid streams which are, for example, conducted in counterflow to one another, for example, a warm compressed air stream and one or more cold fluid streams or a cryogenic liquid air product and one or more warm or warmer but possibly also even cryogenic fluid streams. A heat exchanger can be formed from one or more heat exchanger sections connected in parallel and/or serially, e.g., from one or more plate heat exchanger blocks. It is, for example, a plate fin heat exchanger. Such a heat exchanger has “passages” which take the form of fluid channels separated from one another and having heat exchange surfaces, and which are connected together in parallel and separated by other passages to form “passage groups.” The characteristic of a heat exchanger is that at one time heat is exchanged therein between two mobile media, namely at least one fluid stream to be cooled and at least one fluid stream to be heated.

A “condenser evaporator” refers to a heat exchanger in which a condensing fluid stream enters into indirect heat exchange with an evaporating fluid stream. Each condenser evaporator has a liquefaction chamber and an evaporation chamber. The liquefaction and evaporation chambers have liquefaction or evaporation passages. Condensation (liquefaction) of the condensing fluid stream is carried out in the liquefaction chamber, and evaporation of the evaporating fluid stream is carried out in the evaporation chamber. The evaporation and liquefaction chambers are formed by groups of passages, which are in a heat-exchanging relationship with one another.

The present invention comprises the low-temperature fractionation of air according to the so-called SPECTRA process, as described, inter alia, in EP 2 789 958 A1 and the further patent literature cited therein. In its simplest form, this process is a single-column process. However, in the context of the present invention, this is not the case, because here, in addition to an air-fed rectification column (“first” rectification column), a rectification column fed from the first rectification column and used for oxygen extraction (“second” rectification column) is used.

While the SPECTRA process was originally intended to provide gaseous nitrogen at the pressure level of the first rectification column, the use of a second rectification column of the type explained enables the additional extraction of pure oxygen.

As in other processes for the low-temperature fractionation of air, compressed and pre-purified air is also cooled in the SPECTRA process to a temperature suitable for rectification. It can thereby be partially liquefied. The air is rectified at the typical pressure of a high-pressure column, as explained at the outset, yielding the tops gas enriched in nitrogen in comparison to atmospheric air and a liquid bottoms liquid enriched in oxygen in comparison to atmospheric air.

A return flow of the first rectification column used for this purpose is provided in a heat exchanger by condensing tops gas of the first rectification column, more precisely a portion of this tops gas. In this heat exchanger, a condenser evaporator (“first” condenser evaporator), fluid, which is likewise taken from the first rectification column, is used for cooling and thereby evaporated or partially evaporated. Further tops gas may be provided as a nitrogen-rich product.

In the variant of the SPECTRA process used according to the invention, two material streams (“first” and “second” material streams) are used in the first condenser evaporator, wherein the first material stream is formed using liquid taken from the first rectification column with a first oxygen content and the second material stream is formed using liquid taken from the first rectification column with a second, higher oxygen content. The liquid used to form the first material stream can be taken from the first rectification column from an intermediate tray or from a liquid retention device. The liquid used to form the second material stream can in particular be at least a portion of the liquid bottoms product of the first rectification column. The first and second material streams are the aforementioned fluid which is used in the first condenser evaporator for cooling and for condensing the corresponding portion of tops gas of the first rectification column.

In general, in a SPECTRA process, after being used in the first heat exchanger for cooling, the first material stream can be at least partially compressed by means of a cold compressor and returned to the first rectification column. This is also the case in the context of the present invention. In a SPECTRA process, after being used in the first heat exchanger for cooling, the second material stream may be at least partially expanded and discharged from the air fractionation plant as a so-called residual gas mixture. For the compression of the first material stream (or a corresponding portion), one or more compressors can be used which are coupled to one or more expansion machines in which the expansion of the second material stream (or a corresponding portion) is carried out. It is understood that only portions of the first or second material stream may also in each case be compressed or expanded in the correspondingly coupled units. An expansion machine that is not coupled to a corresponding compressor can, if present, be braked in particular mechanically and/or by generator. Braking is also possible in the case of an expansion machine that is coupled to a compressor.

For example, a compressor that is coupled to one of two expansion machines arranged in parallel can be used. If only one expansion machine is used, the compressor can be coupled thereto. The wording, used below only for reasons of clarity, according to which “a” compressor is coupled to “an” expansion machine, does not preclude the use of a plurality of compressors and/or expansion machines in any mutual coupling. However, the compressor or compressors described do not have to be driven, in particular not exclusively, by means of the one or more expansion machines mentioned. Conversely, the compressor or compressors also do not have to take up all of the work released during expansion. As also illustrated below by way of example, a supporting or exclusive drive can also be effected, for example, by using an electric motor, or a brake can be interposed between the expansion machine(s) and the compressor(s).

The compressor or compressors are one or more cold compressors, since the compressor or compressors are supplied with the first fluid stream despite its routing through the first condenser evaporator and an optionally subsequent further heating at a low temperature level.

Advantages of the Invention

In the SPECTRA processes just explained with oxygen extraction, a condenser evaporator (“second” condenser evaporator) is typically present in the lower region of the second rectification column and is used to bring bottoms liquid in the second rectification column to boil. This condenser evaporator is conventionally operated with air (feed air) that is compressed (at least) in the main air compressor and cooled in the main heat exchanger, and which is supplied to the first rectification column. In particular, this feed air can also be air that is initially present in gaseous form and is liquefied in the second condenser evaporator before it is fed into the first rectification column. It is a portion of the feed air supplied overall to the first rectification column. Further (gaseous) feed air can be fed into the first rectification column without any corresponding liquefaction.

The costs for obtaining high-purity liquid or gaseous oxygen products (LOX, GOX; in the high-purity state, also referred to as UHPLOX or UHPGOX) are comparatively high in known SPECTRA processes. The reason for this is, on the one hand, a relatively high outlay on equipment and, on the other hand, an additional energy requirement associated therewith which can considerably influence the efficiency of the overall process.

The reason for the high specific energy requirement is mainly that in the aforementioned second condenser evaporator in the lower region of the second rectification column, the explained “heating” is realized to a large extent by the aforementioned condensation of the portion of the gaseous feed air. Although this (liquefied) partial air stream is subsequently (after its evaporation) used for generating cooling capacity or for driving the cold compressor(s) used, in that a corresponding quantity of fluid is taken as the second material stream from the first rectification column, said partial air stream does however not participate further in the rectification process in the first rectification column. This leads to a strong reduction in the product yield of nitrogen, since a return flow, lacking here, to the first rectification column must be generated by a fractionation of additional air. The high driving temperature difference in the second condenser evaporator in the lower region of the second rectification column (condensation of the air here takes place at the highest pressure in the rectification column system) leads to additional thermodynamic losses in the process. Especially in the case of relatively large quantities of UHPGOX or UHPLOX products, these disadvantages have a great effect on the power consumption of the main air compressor.

The present invention is based on the finding that a process of the type explained above can be modified particularly advantageously in that, instead of a feed air stream in the second condenser evaporator in the lower region of the second rectification column, fluid is used which has been evaporated in the manner explained above as part of the “first” or “second” material stream in the first condenser evaporator. In this way, the air previously used for this purpose can be saved, thereby increasing energy efficiency and yield. The condensed fluid can then be treated as explained below.

The invention also opens up particular advantages in that condensation is carried out in the second condenser evaporator at a pressure level at which evaporation of the corresponding fluid in the first condenser evaporator was previously carried out. In this way, recompression can be dispensed with and the condensed gas or the condensate formed can be brought to the required pressure by means of a pump. In contrast to a gas compressor, the operation of a pump is much more reliable and its provision is significantly more cost-effective.

Overall, in the wording of the claims, the present invention proposes a process for the low-temperature fractionation of air in which an air fractionation plant having a first rectification column and a second rectification column is used. The first rectification column is operated at a first pressure level and the second rectification column is operated at a second pressure level below the first pressure level.

Such first and second pressure levels are typical pressure levels, such as are also used in conventional air fractionation plants, in particular in SPECTRA plants with oxygen extraction. The first pressure level may, in particular, be 7 to 14 bar, and the second pressure level may, in particular, be 1.2 to 5 bar. The second pressure level may generally also be 1 to 4 bar. These are in each case absolute pressures at the top of the respective rectification columns. The first rectification column and the second rectification column can, in particular, be arranged next to one another and are typically not combined with one another in the form of a double column, wherein here a “double column” is understood to mean a separator consisting of two rectification columns and designed as a structural unit, in which column jackets of the two rectification columns are connected, in particular welded, to one another without lines, i.e., directly. However, no fluidic connection needs to be established by this direct connection alone.

The first rectification column used in the context of the present invention and the second rectification column used in the context of the present invention have already been described in detail above with reference to the SPECTRA process. The second rectification column can, in particular, be an oxygen column.

Atmospheric air, which has been compressed and then cooled, is here supplied to the first rectification column. If necessary, corresponding air can be supplied to the first rectification column in the form of a plurality of material streams which can be treated differently and optionally have been previously routed through further apparatuses. In contrast, air is not typically supplied to the second rectification column. The second rectification column is fed from the first rectification column or no material streams that have not already been taken from the first rectification column or formed from such material streams are typically supplied to the second rectification column.

As customary in a SPECTRA process, and in the context of the present invention as well, tops gas of the first rectification column is obtained as a nitrogen product and discharged from the air fractionation plant, and bottoms liquid of the second rectification column is obtained as an oxygen product and discharged from the air fractionation plant. This does not preclude that further fluids, for example tops gas of the second rectification column, are also discharged from the air fractionation plant and, for example, released into the atmosphere. Fluids otherwise discharged as nitrogen or oxygen products can within the scope of the present invention also be discharged in certain proportions, for example as purge streams or as a further liquid nitrogen product after condensation of tops gas of the first rectification column.

In the first condenser evaporator, in the context of the present invention, a first and a second material stream are separately subjected to evaporation below the first pressure level. The evaporation pressures in the first condenser evaporator are in particular 3.5 to 7.5 bara (bar absolute pressure; the values can represent exact or even approximate values) depending on the first pressure level. The fluid that is to be evaporated here, which was taken from the first rectification column, is therefore correspondingly expanded. Further tops gas of the first rectification column, which is not provided as a gaseous nitrogen product, is condensed in the first condenser evaporator and returned to the first rectification column as a return flow. A proportion of corresponding condensate can also be discharged as the further liquid nitrogen product mentioned, in particular after supercooling against itself.

In the context of the present invention, the first material stream evaporated in the first condenser evaporator is formed using liquid taken from the first rectification column with a first oxygen content, and the second material stream is formed using liquid taken from the first rectification column with a second oxygen content above the first oxygen content. Further explanations relating to such liquids have already been given. The liquid with the first, lower oxygen content is in particular liquid that is extracted at an intermediate tray or separating tray of the first rectification column or of a corresponding liquid retention device. The liquid with the second, higher oxygen content is in particular bottoms liquid of the first rectification column.

In the context of the present invention, after its evaporation or partial evaporation in the first condenser evaporator, gas of the first material stream is partially or completely subjected to recompression to the first pressure level and fed into the first rectification column, and gas of the second material stream is, after its evaporation or partial evaporation, subjected to expansion in the first condenser evaporator and discharged from the air fractionation plant.

In this case, the wordings “gas of the first material stream” and “gas of the second material stream” are in particular also to encompass the fact that the entire gas of the first and second material streams is used in the manner explained if a certain portion is not used in other ways, as explained below, in embodiments of the present invention. The wordings “gas of the first material stream” and “gas of the second material stream” are in other words thus intended to denote all or a portion of the corresponding gas but do not preclude that there are further uses within the scope of the invention.

The second rectification column is equipped with or at least thermally coupled to the second condenser evaporator, wherein the second condenser evaporator is designed or provided in particular in a bottoms region of the second rectification column and is in particular partially immersed in a liquid bath forming in the bottoms region. The bottoms liquid of the second rectification column is here evaporated in the second condenser evaporator. In particular, liquid can also be cooled (supercooled) in the second condenser evaporator, by means of which liquid the second rectification column is fed from the first rectification column.

According to the invention, after its evaporation or partial evaporation in the first condenser evaporator, gas of the first or second material stream is subjected to condensation in the second condenser evaporator and, in particular, after a corresponding increase in pressure in the liquid state, is fed at least in part to the liquid taken from the first rectification column with the first or second oxygen content and used in the formation of the first or second material stream, or fed instead into a lower region of the first rectification column. It also applies here that, when “gas of the first or second material stream” is mentioned, other gas of the first or second material stream or the respectively unaffected material stream can be used partially or completely in the manner explained (recompressed and returned to the first rectification column or expanded and discharged).

Advantages of the present invention have already been addressed. The advantage of the interconnection provided within the scope of the present invention is in particular that, in order to obtain identical products, approximately 3% less feed air (or approximately 3% less energy) is required, wherein, for example, a quantity of 29,300 standard cubic meters per hour of pressurized nitrogen (PGAN) and 700 standard cubic meters per hour of high-purity liquid oxygen (UHPLOX) can be provided. This can be attributed in particular to the fact that, within the scope of the present invention, no air is used to heat the condenser evaporator and the disadvantages explained above are thus overcome. Within the context of the present invention, the specific energy requirement is reduced because all of the air used takes part in the rectification process in the first rectification column and the product yield of nitrogen is thus increased.

The two alternatives of the invention (relating to the first and the second material streams) are in particular that on the one hand after its evaporation in the first condenser evaporator, a first portion of the first material stream is subjected to recompression to the first pressure level, and that after its evaporation in the first condenser evaporator, a second portion of the first material stream is subjected to condensation in the second condenser evaporator, and that on the other hand after its evaporation in the first condenser evaporator, a first portion of the second material stream is subjected to work-performing expansion, and that after its evaporation in the first condenser evaporator, a second portion of the second material stream is subjected to condensation in the second condenser evaporator.

In both cases, it is provided according to the invention that the gas of the first or second material stream, which is subjected to condensation in the second condenser evaporator after evaporation or partial evaporation in the first condenser evaporator, is subjected to condensation in the second condenser evaporator at a pressure level at which it was previously subjected to evaporation or partial evaporation in the first condenser evaporator.

For this purpose, the gas of the first or second material stream, which after evaporation or partial evaporation in the first condenser evaporator is subjected to condensation in the second condenser evaporator, is transferred into the second condenser evaporator in particular using a gas line that without a compressor couples the first condenser evaporator and the second condenser evaporator. In particular, a coupling realized directly and without pressure-influencing devices can be provided in this case.

As also already mentioned above, according to a particularly preferred embodiment of the invention, at least a portion of a condensate formed during condensation in the first condenser evaporator is by use of a pump subjected to an increase in pressure in the liquid state. The pressure increase in the liquid state is in particular carried out to the first pressure level.

At least a portion of the condensate formed during condensation in the first condenser evaporator and subjected to the increase in pressure in the liquid state can here be fed in the aforementioned first alternative to the liquid with the first or second oxygen content that is taken from the first rectification column and used in the formation of the first or second material stream, or be fed in the other alternative likewise mentioned into a lower region of the first rectification column.

According to the invention, as just mentioned in other words, in one embodiment, gas of the first or second material stream after its evaporation or partial evaporation in the first condenser evaporator can thus be subjected to condensation in the second condenser evaporator, and after a corresponding increase in pressure in the liquid state, can be fed at least partially into a lower region of the first rectification column. Such a “lower region” can advantageously be a position at which a first material stream evaporated in the first condenser evaporator or its liquid is taken from the first rectification column.

In principle, the pressurized nitrogen product from the first rectification column could also be used to heat the second condenser evaporator and be subjected to corresponding condensation. However, in order to not impair its purity, a correspondingly contaminant-free pump would have to be used for further conveyance of the liquid formed. Due to the outlay associated therewith, this is extremely disadvantageous in comparison to the solutions proposed according to the invention.

As known in this respect with SPECTRA processes, one or more compressors can also be provided within the scope of the present invention for the recompression of the gas of the first material stream after its evaporation or partial evaporation in the first condenser evaporator; and for the expansion of the gas of the second material stream after its evaporation or partial evaporation in the first condenser evaporator, one or more expansion machines may be provided which is or are coupled to the one or more compressors. Further details of SPECTRA processes have already been explained in general terms above.

With the process according to the invention, a tops gas of the first rectification column, and thus a nitrogen product, can be obtained with a content of respectively less than 1 ppb oxygen, carbon monoxide, and/or hydrogen and a content of less than 10 ppm argon on a volume basis. In particular, the bottoms liquid of the second rectification column can have a content of less than 10 ppb argon and/or 5 ppm methane on a volume basis and otherwise consist substantially of oxygen.

The first rectification column can be operated in such a way that the first pressure level is 7 to 14 bar absolute pressure, in particular 8 to 12 bar, and that the second pressure level is 1.2 to 5 bar, in particular 2 to 4 bar, absolute pressure.

Advantageously, all cooled compressed air to be fractionated in the process is fed into the first rectification column in gaseous form.

The present invention also extends to an air fractionation plant which has a first rectification column, a second rectification column, a first condenser evaporator, and a second condenser evaporator and is configured to feed the first rectification column with air and operate it at a first pressure level and to feed the second rectification column from the first rectification column and operate it at a second pressure level below the first pressure level. The air fractionation plant is also configured to obtain tops gas of the first rectification column as a nitrogen product and to discharge it from the air fractionation plant and to obtain bottoms liquid of the second rectification column as an oxygen product and to discharge it from the air fractionation plant; in the first condenser evaporator to subject a first and a second material stream below the first pressure level to evaporation and to condense further tops gas of the first rectification column in the first condenser evaporator and to return it to the first rectification column as a return flow; to form the first material stream using liquid taken from the first rectification column with a first oxygen content, and to form the second material stream using liquid taken from the first rectification column with a second oxygen content above the first oxygen content; to partially or completely subject gas of the first material stream, after its evaporation or partial evaporation in the first condenser evaporator, to recompression to the first pressure level and to feed it into the first rectification column, and to subject gas of the second material stream, after its evaporation or partial evaporation in the first condenser evaporator, to expansion and to discharge it from the air fractionation plant; and to evaporate bottoms liquid of the second rectification column in the second condenser evaporator.

In the proposed plant, means are provided which are configured to subject gas of the first or second material stream, after its evaporation or partial evaporation in the first condenser evaporator, to condensation in the second condenser evaporator and to feed at least a portion of it to the liquid taken from the first rectification column with the second oxygen content and used in the formation of the second material stream or to feed it into a lower region of the second rectification column.

For the recompression of the gas of the first material stream after its evaporation or partial evaporation in the first condenser evaporator, one or more compressors are provided, and for the expansion of the gas of the second material stream after its evaporation or partial evaporation in the first condenser evaporator, one or more expansion machines mechanically coupled to the one or more compressors are provided.

According to the invention, the first condenser evaporator and the second condenser evaporator are arranged and in particular are coupled to one another without a compressor via a gas line in such a way that the gas of the first or second material stream, which after evaporation or partial evaporation in the first condenser evaporator is subjected to condensation in the second condenser evaporator, is subjected to condensation in the second condenser evaporator at a pressure level at which it was previously subjected to evaporation or partial evaporation in the first condenser evaporator.

Reference is made to the above explanations of the process according to the invention and its embodiments for further features and advantages of the air fractionation plant according to the invention which is configured in particular for carrying out a process as explained above in different embodiments and has corresponding means realized in terms of devices.

The invention is described in more detail hereafter with reference to the accompanying drawings, which illustrate preferred embodiments of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an air fractionation plant according to an embodiment of the invention.

FIG. 2 shows an air fractionation plant according to an embodiment of the invention.

FIG. 3 shows an air fractionation plant according to an embodiment of the invention.

In the figures, elements corresponding functionally or structurally to one another are indicated by identical reference signs and only for the sake of clarity are not repeatedly explained below.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an air fractionation plant 100 in the form of a schematic plant diagram. The central component is a distillation column system having a first rectification column 11, a second rectification column 12, a first condenser evaporator 111, and a second condenser evaporator 121. The first rectification column 11 is operated at a first pressure level, and the second rectification column 12 is operated at a second pressure level below the first pressure level.

By means of a main air compressor 1 of the air fractionation plant 100, air is sucked in from the atmosphere A via a filter (not separately designated) and compressed. After cooling in an aftercooler (likewise not designated separately) downstream of the main air compressor 1, the feed air stream a formed in this way is further cooled in a direct contact cooler 2 operated with water W. The feed air stream a is then subjected to cleaning in an adsorber unit 3. For further explanations in this context, reference is made to the technical literature, for example in connection with FIG. 2.3A in Haring (see above).

After cooling in the main heat exchanger 4, the feed air stream a is fed into the first rectification column 11. In a conventional process, a portion of the feed air stream a would be fed into the first rectification column 11, whereas a further portion would be routed through the second condenser evaporator 121, which is arranged in a lower region of the second rectification column 12, and evaporated by means of the bottoms liquid of the second rectification column 12. This further portion would be condensed in the second condenser evaporator 121 and then likewise fed into the first rectification column 11. As mentioned, this is not the case in the embodiments of the invention.

Tops gas of the first rectification column 11 is discharged from the air fractionation plant 300 in the form of a material stream d as a nitrogen product B or sealing gas C. In contrast, bottoms liquid of the second rectification column 12 is discharged in the form of a material stream e as an oxygen product D. It is also possible, for example, to feed into so-called run tanks for later evaporation for the provision of an internally compressed oxygen product D.

In the first condenser evaporator 111, a first material stream g and a second material stream h below the first pressure level (for this purpose, a corresponding expansion in particular takes place in valves which are not designated separately) are subjected to evaporation. Further tops gas of the first rectification column 11 is condensed in the form of a material stream i in the first condenser evaporator 111 and returned to the first rectification column 11 as a return flow. As illustrated here in the form of a material stream k, a portion can also be supercooled in a supercooler 5 and provided as liquid nitrogen F. A material stream I heated thereby is treated as explained in more detail below. A further discharge in the form of a purge stream m or P may also be provided.

The first material stream g is formed using liquid taken from the first rectification column 11 with a first oxygen content, and the second material stream h is formed using liquid (in particular bottoms liquid) taken from the first rectification column 11 with a second oxygen content above the first oxygen content.

After its evaporation or partial evaporation in the first condenser evaporator 111, gas of the first material stream g is subjected in a compressor 6 to recompression to the first pressure level and fed into the first rectification column 11. A portion indicated by a dashed line can also be returned to compression in the compressor 6. A portion of the material stream g can also be discharged into the atmosphere A in the form of a material stream n.

After its evaporation or partial evaporation in the first condenser evaporator 111, gas of the second material stream h is subjected to parallel further expansion in expansion machines 7 and 8, combined with tops gas, which is taken in the form of a material stream o from the second rectification column 12, and, after heating in the main heat exchanger 4, used as regeneration gas in the adsorber unit 3 or discharged to the atmosphere A and thus discharged from the air fractionation plant 300.

The expansion machine 7 is coupled to the compressor 6, and the expansion machine 8 is coupled to a generator G. In each case, a different number of corresponding machines or a different type of coupling may also be used. An (oil) brake (not separately designated) may also be provided.

The second rectification column 12 is fed with a side stream p of the first rectification column 11, which is passed through the second condenser evaporator 121 and fed into the second rectification column in an upper region. In addition, gas of the second material stream h after its evaporation or partial evaporation in the first condenser evaporator 111 is conducted as a partial stream b through the condenser evaporator 121 and subjected to condensation. A correspondingly formed liquid, further designated by b, has its pressure increased by means of a pump 9 and is subsequently recombined with the second material stream h prior to its evaporation.

In the otherwise substantially identical or comparable air fractionation plant 200 according to FIG. 2, the liquid formed in the condenser evaporator 121 by condensation of gas of the second material stream h in the form of the material stream b also has its pressure increased by means of the pump 9 but is then fed into the first rectification column 11 in a lower region.

A partial stream of the first material stream g can also be used accordingly, as illustrated in FIG. 3 with a material stream c. The air fractionation plant 300 according to FIG. 3 can otherwise be substantially identical or comparable. The liquid formed in the second condenser evaporator 121 by condensation of gas of the first material stream g has its pressure increased by means of the pump 9 and is then recombined with the first material stream g before evaporation in the first condenser evaporator 111. 

1-13. (canceled)
 14. A process for the low-temperature fractionation of air, in which an air fractionation plant having a first rectification column, a second rectification column, a first condenser evaporator, and a second condenser evaporator is used, wherein the process comprises that the first rectification column is fed with air and operated at a first pressure level, and the second rectification column is fed from the first rectification column and operated at a second pressure level below the first pressure level, wherein tops gas of the first rectification column is obtained as a nitrogen product and discharged from the air fractionation plant, and bottoms liquid of the second rectification column is obtained as an oxygen product and discharged from the air fractionation plant, a first and a second material stream below the first pressure level are subjected to evaporation in the first condenser evaporator, and further tops gas of the first rectification column is condensed in the first condenser evaporator and returned to the first rectification column as a return flow, the first material stream is formed using liquid taken from the first rectification column with a first oxygen content, and the second material stream is formed using liquid taken from the first rectification column with a second oxygen content above the first oxygen content, gas of the first material stream after its evaporation or partial evaporation in the first condenser evaporator is partially or completely subjected to recompression to the first pressure level and fed into the first rectification column, and gas of the second material stream after its evaporation or partial evaporation in the first condenser evaporator is subjected to work-performing expansion and discharged from the air fractionation plant, and bottoms liquid of the second rectification column is evaporated in the second condenser evaporator, and gas of the first or second material stream after its evaporation or partial evaporation in the first condenser evaporator is subjected to condensation in the second condenser evaporator and fed at least in part to the liquid taken from the first rectification column with the first or second oxygen content and used in the formation of the first or second material stream, or fed into a lower region of the first rectification column, wherein the gas of the first or second material stream, which is subjected to condensation in the second condenser evaporator after its evaporation or partial evaporation in the first condenser evaporator, is subjected to condensation in the second condenser evaporator at a pressure level at which it was previously subjected to evaporation or partial evaporation in the first condenser evaporator.
 15. The process according to claim 14, in which a first portion of the first material stream after its evaporation in the first condenser evaporator is subjected to recompression to the first pressure level, and in which a second portion of the first material stream after its evaporation in the first condenser evaporator is subjected to condensation in the second condenser evaporator.
 16. The process according to claim 14, in which a first portion of the second material stream after its evaporation in the first condenser evaporator is subjected to work-performing expansion, and in which a second portion of the second material stream after its evaporation in the first condenser evaporator is subjected to condensation in the second condenser evaporator.
 17. The process according to claim 14, in which one or more compressors is or are provided for the recompression of the gas of the first material stream after its evaporation or partial evaporation in the first condenser evaporator, and in which, for the expansion of the gas of the second material stream after its evaporation or partial evaporation in the first condenser evaporator, one or more expansion machines are provided which is or are coupled to the one or more compressors.
 18. The process according to claim 14, in which the gas of the first or second material stream, which after evaporation or partial evaporation in the first condenser evaporator is subjected to condensation in the first condenser evaporator, is transferred into the second condenser evaporator using a gas line that without a compressor couples the first condenser evaporator and the second condenser evaporator.
 19. The process according to claim 14, in which at least a portion of a condensate formed during condensation in the first condenser evaporator is by use of a pump subjected to an increase in pressure in the liquid state.
 20. The process according to claim 19, in which the pressure increase to the first pressure level is carried out in the liquid state.
 21. The process according to claim 14, in which the tops gas of the first rectification column has a content of in each case less than 1 ppb oxygen, carbon monoxide, and/or hydrogen and a content of less than 10 ppm argon on a volume basis.
 22. The process according to claim 14, in which the bottoms liquid of the second rectification column has a content of less than 10 ppb argon and/or 5 ppm methane on a volume basis.
 23. The process according to claim 14, in which the first pressure level is 7 to 14 bar absolute pressure and in which the second pressure level is 1.2 to 5 bar absolute pressure.
 24. The process according to claim 14, in which all of the cooled compressed air to be fractionated in the process is fed into the first rectification column in gaseous form.
 25. An air fractionation plant having a first rectification column, a second rectification column, a first condenser evaporator, and a second condenser evaporator and configured to feed the first rectification column with air and to operate it at a first pressure level, and to feed the second rectification column from the first rectification column and to operate it at a second pressure level below the first pressure level, to obtain tops gas of the first rectification column as a nitrogen product and discharge it from the air fractionation plant, and to obtain bottoms liquid of the second rectification column as an oxygen product and discharge it from the air fractionation plant, to subject a first and a second material stream below the first pressure level to evaporation in the first condenser evaporator, and to condense further tops gas of the first rectification column in the first condenser evaporator and to return it to the first rectification column as a return flow, to form the first material stream using liquid taken from the first rectification column with a first oxygen content, and to form the second material stream using liquid taken from the first rectification column with a second oxygen content above the first oxygen content, to partially or completely subject gas of the first material stream, after its evaporation or partial evaporation in the first condenser evaporator, to recompression to the first pressure level and to feed it into the first rectification column, and to subject gas of the second material stream, after its evaporation or partial evaporation in the first condenser evaporator, to expansion and to discharge it from the air fractionation plant, to evaporate bottoms liquid of the second rectification column in the second condenser evaporator, and to subject gas of the first or second material stream, after its evaporation or partial evaporation in the first condenser evaporator, to condensation in the second condenser evaporator and to feed it at least in part to the liquid taken from the first rectification column with the first or second oxygen content and used in the formation of the first or second material stream, or to feed it into a lower region of the first rectification column, wherein for the recompression of the gas of the first material stream after its evaporation or partial evaporation in the first condenser evaporator, one or more compressors are provided, and for the expansion of the gas of the second material stream after its evaporation or partial evaporation in the first condenser evaporator, one or more expansion machines mechanically coupled to the one or more compressors are provided, wherein the first condenser evaporator and the second condenser evaporator are arranged in such a way that the gas of the first or second material stream, which is subjected to condensation in the second condenser evaporator, is subjected to condensation at a pressure level at which it was previously subjected to evaporation in the first condenser evaporator.
 26. Air fractionation plant according to claim 25, having means which are configured to carry out a process for the low-temperature fractionation of air, in which an air fractionation plant having a first rectification column, a second rectification column, a first condenser evaporator, and a second condenser evaporator is used, wherein the process comprises that the first rectification column is fed with air and operated at a first pressure level, and the second rectification column is fed from the first rectification column and operated at a second pressure level below the first pressure level, wherein tops gas of the first rectification column is obtained as a nitrogen product and discharged from the air fractionation plant, and bottoms liquid of the second rectification column is obtained as an oxygen product and discharged from the air fractionation plant, a first and a second material stream below the first pressure level are subjected to evaporation in the first condenser evaporator, and further tops gas of the first rectification column is condensed in the first condenser evaporator and returned to the first rectification column as a return flow, the first material stream is formed using liquid taken from the first rectification column with a first oxygen content, and the second material stream is formed using liquid taken from the first rectification column with a second oxygen content above the first oxygen content, gas of the first material stream after its evaporation or partial evaporation in the first condenser evaporator is partially or completely subjected to recompression to the first pressure level and fed into the first rectification column, and gas of the second material stream after its evaporation or partial evaporation in the first condenser evaporator is subjected to work-performing expansion and discharged from the air fractionation plant, and bottoms liquid of the second rectification column is evaporated in the second condenser evaporator, and gas of the first or second material stream after its evaporation or partial evaporation in the first condenser evaporator is subjected to condensation in the second condenser evaporator and fed at least in part to the liquid taken from the first rectification column with the first or second oxygen content and used in the formation of the first or second material stream, or fed into a lower region of the first rectification column, wherein the gas of the first or second material stream, which is subjected to condensation in the second condenser evaporator after its evaporation or partial evaporation in the first condenser evaporator, is subjected to condensation in the second condenser evaporator at a pressure level at which it was previously subjected to evaporation or partial evaporation in the first condenser evaporator. 